Semiconductor integrated circuit devices, which are typically formed from silicon or gallium arsenide, are conventionally enclosed in plastic packages. The elements of such a package include a metal leadframe, an integrated circuit device, bonding material to attach the integrated circuit device to the leadframe, bond wires or other connectors that conductively connect metal pads on the integrated circuit device to individual leads of the leadframe, and a hard plastic encapsulant material that covers the other components and forms the exterior of the package.
FIG. 1 is a conventional metal leadframe 10. Although not shown, several identical leadframes 10 are formed in an array from a single sheet of metal and each leadframe of the array is processed in parallel.
Leadframe 10 includes a central metal die pad 11 (shown by horizontal hatching) and radiating metal leads 12 adjacent to die pad 11. An opposite end of each lead 12 is connected to the peripheral frame of the leadframe (not shown). A dam bar (not shown) also crosses and connects the leads. The frame and dam bar are removed during a trim and form step. The portion of each lead 12 that is within the dashed line is plated with silver. The perimeter of die pad 11 also may be plated. A ring of an adhesive polyimide tape 13 (shown by diagonal hatching) crosses and connects each of the leads 12 at a midpoint and serves to stabilize the leads during processing. Four tie bars 14 support die pad 11 and connect it to the peripheral dam bar and frame (not shown). Five leads 15 are connected to die pad 11 for support. Die pad 11 is downset, as indicated by downset marks 16 on tie bars 14 and leads 15.
An alternative prior art embodiment of a die pad shown is in FIG. 2. Metal die pad 20 is surrounded by a ring 21. Ring 21 is connected by sixteen metal connectors 22 to die pad 20. Tie bars 23 connect die pad 20 to the remainder of the peripheral frame of the leadframe (not shown). This configuration facilitates spacing of the leads (not shown) and provides an encapsulant locking feature.
In a completed package, the integrated circuit device is mounted on the die pad. Typically, integrated circuit devices require connection to power voltage source, sometimes denoted "Vdd," and a ground voltage source, sometimes denoted "Vss." A lead of the package typically is dedicated to supply Vdd. Sometimes, a second dedicated lead provides Vss. The use of dedicated leads for power and ground voltages, however, limits the flexibility of the package because the user of the package, for example, a chip manufacturer or computer manufacturer, cannot vary which leads are to be used for connection to input and output ("I/O") signals and which leads are to be used for connection to Vdd and Vss.